Since it was invented in the early 1970's, ion mobility spectrometry (IMS) has been developed into a powerful analytical tool used in a variety of applications. There are three major forms of this instrument including independent chemical detection systems, chromatographic detectors, or hyphenated IMS mass spectrometry (MS) systems. As an independent detection system, IMS qualitatively and quantitatively detects substances in different forms relying on its capability to ionize the target substance, to separate the target substance from background based on interactions with a drift gas (i.e. a carrier gas), and to detect the substance in its ionized form. As a chromatographic detector, IMS acquires multiple ion mobility spectra of chromatographically separated substances. In combined IMS-MS systems, IMS is used as a separation method to isolate target substances before mass analysis. However, the resolution of IMS is generally consider low, often regulating such devices to qualitative use or use in environments with low levels of interferants with respect to the substances of interest.
The basic common components of an IMS system consist of an ionization source, a drift tube that includes a reaction region, an ion shutter grid, a drift region, and an ion detector. In gas phase analysis the sample to be analyzed is introduced into the reaction region by an inert carrier gas, ionization of the sample is often completed by passing the sample through a reaction region and/or a radioactive 63Ni source. The ions that are formed are directed toward the drift region by an electric field applied to drift rings that establish the drift region, and a narrow pulse of ions is then injected into, and/or allowed to enter, the drift region via an ion shutter grid. Once in the drift region, ions of the sample are separated based upon their ion mobilities and there arrival time at a detector is an indication of ion mobility which can be related to ion mass. However, it is to be understood that ion mobility is not only related to ion mass, but rather is fundamentally related to the ion-drift gas interaction potential which is not solely dependent on ion mass.
Ion mobility spectrometers (IMS) have become a common tool for detecting trace amounts of chemical and/or biological molecules. Compared to other spectrometric chemical analysis technologies, e.g., mass spectrometry, IMS is a relatively low resolution technique. The IMS advantages of very high sensitivity, small size, low power consumption, and ambient pressure operation are in some cases completely offset, or at a minimum, reduced by the lack of sufficient resolution to prevent unwanted responses to interfering chemical and/or biological molecules. The false positives that result can range from minor nuisances in some scenarios to major headaches in others. Interfering chemical and/or biological molecules can have very similar ion mobilities which in turn can significantly limit detecting and identifying low levels of the targeted chemical and/or biological molecules in the sample.
The present state of the art ion mobility spectrometers lack the ability to directly reduce the occurrence of interfering chemical and/or biological molecules in a sample's analysis. It is the purpose of this invention to overcome these obstacles by making the use of a cross-directional gas flow in a drift tube and/or using a segmented drift tube for pre-separation.